In friction stir welding, as a probe (a rod-shaped protrusion) installed at a tip section of a friction stir welding tool is rotated and pushed to a junction of workpieces to be merged into the workpieces, frictional heat is generated at the junction of the workpieces to soften the workpieces, causing plastic flow of the peripheries of the junction and a rotational force of the friction stir welding tool stir mixes the plastic flow to integrally weld the plurality of workpieces. At the present time, friction stir welding is mainly used to weld aluminum thin plates (a thickness of 10 mm or less) in which a welding reaction force generated upon the welding is small.
As a conventional friction stir welding apparatus used for the friction stir welding, an apparatus including guide rails installed at both sides of a surface plate on which a workpiece is placed, a portal frame movably installed on the guide rails spanning the surface plate, a base movably cantilever-supported at the portal frame in a widthwise direction of the surface plate, a welding apparatus main body movably cantilever-supported at the base in an upward/downward direction, and a friction stir welding tool rotatably attached to a lower end section of the welding apparatus main body is proposed (for example, see Japanese Patent Application, First publication No. 2004-195549).
In addition, as another example of the conventional friction stir welding apparatus, an apparatus in which a welding apparatus main body having a friction stir welding tool and a rotary driving unit configured to rotatably drive the friction stir welding tool and a displacement driving unit configured to move the friction stir welding tool in an upward/downward direction is mounted on a self-propelled vehicle body including wheels and a wheel rotating unit configured to rotatably drive the wheels is proposed (for example, see Japanese Patent Application, First publication No. 2005-186084).